Re: [lwip-users] lwip_standard_chksum()

[Timmy Brolin]

Yes, I think I might have a better idea.
This solution would even improve performance on machines which support 
unaligned memory accesses.

First test if the address is unaligned.
If aligned:
  Do a normal 16bit checksum.
else:
  Read first byte (and add to checksum).
  Then continue by doing a normal 16bit checksum.
  At the end we need to byte-swap the result to compensate for the 
off-by-one-byte 16bit additions.

I think this should work, and significantly improve performance for all 
non 8-bit architectures since it prevents unaligned memory accesses. 
(unaligned memory accesses usually take about twice as long as aligned 
accesses)

I do not like the idea of putting a 8bit checksum computation in the 
standard code.

Regards,
Timmy Brolin

Christiaan Simons wrote:

> Hi all,
>
> It seems lwip_standard_chksum() from inet.c 1.22 and up
> is disfunctional for the c16x and likely other 16 bit mcu's.
>
> As far as I understand the problem is in that the source data
> (void *dataptr ... ) can be at an odd (unaligned) address.
>
> This was "solved" by a patch posted by Peter Jolasson
> a few months ago and was partially committed by Leon.
> (a crude/dubious runtime test for odd addresses was removed)
>
> IMHO we need to change our reference implementation
> to be correct for any architecture.
>
> I've got some ideas:
>
> * Add octets (byte-by-byte) instead of 16 bit words as shown in RFC1071 p5.
> This will lower the performance for >8 bit architectures (a lot),
> but will be alignment independant.
>
> * Revert to the pre 1.22 routine, and enforce compile-time alignment by
> changing
> the function argument "void* dataptr" into "u16_t* dataptr" _and_ ensure
> that  pbuf->payload pointers (all args passed to LWIP_CHKSUM) are aligned
> to u16_t.
>
> Anyone got another (better) idea?
>
> Christiaan Simons
>
> Hardware Engineer
> Axon Digital Design
>
> +31 (0)13 511 66 66
> +31 (0)13 511 41 51
>
> http://www.axon.tv
>
>
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